(1) Technical Field
The present invention relates to telephone instruments and more particularly to a circuit for lighting a lamp at a subscriber's instrument to alert the subscriber that a message is waiting for them at an associated operator station or message center.
(2) Background Art
To alert a subscriber that a message is waiting, a light at the station is usually lit indicating the presence of a message. A means of light indication is required that typically requires a low current to operate. This current usually is in the order of less than one milliampere. Accordingly a circuit is required which regulates current to a light emitting diode so that excess current is not drawn and the light emitting diode still has sufficient brightness for the desired application.
Historically the message waiting function has been provided mainly by the use of incandescent or neon type lamps. The use of incandescent lamps is quite wasteful of current while neon lamps on the other hand are relatively expensive. Neon lamps, especially of the high brightness type are variable in the firing or breakdown voltage causing some problems also in choosing the proper operating voltage. The advantages of neon lamps are low operating current and low leakage below the firing voltage. However until recently light emitting diodes required relatively more operating current although operating voltage is low. A light emitting diode or "LED" as they are known, that operated at one milliampere or less of current with visible light output, was not available. Most recently the situation has changed and such LED units are now available.
The use of a light emitting diode as the only component of the message waiting indication circuit is unsatisfactory, because of the need to limit current to the device in the operating mode and also to be low in leakage current while in the "off" mode.
Originally attempts were made to utilize only an additional zener diode and a resistor in series with a light emitting diode. Such an arrangement was unable to provide the proper control and regulation of current to the light emitting diode. Another approach was the use of a conventional current source however an excessive amount of reference current was required which amounted to waste or lost current. Again the light emitting diode state-of-the-art was such that as much as possible of the available current must be fed to the light emitting diode. No wasted current could be tolerated. Thus it is obvious that it is desirable to develop a message waiting lamp circuit with low operating current and low leakage at lower cost than prior art neon and incandescent lamps. Since low current light emitting diodes are now available, it is the object of the present invention to provide the proper supporting circuit for such application of light emitting diodes.